US 20060245984 A1
A catalyst element (30) for high temperature applications such as a gas turbine engine. The catalyst element includes a metal substrate such as a tube (32) having a layer of ceramic thermal barrier coating material (34) disposed on the substrate for thermally insulating the metal substrate from a high temperature fuel/air mixture. The ceramic thermal barrier coating material is formed of a crystal structure populated with base elements but with selected sites of the crystal structure being populated by substitute ions selected to allow the ceramic thermal barrier coating material to catalytically react the fuel-air mixture at a higher rate than would the base compound without the ionic substitutions. Precious metal crystallites may be disposed within the crystal structure to allow the ceramic thermal barrier coating material to catalytically react the fuel-air mixture at a lower light-off temperature than would the ceramic thermal barrier coating material without the precious metal crystallites.
1. A catalyst element for a gas turbine engine comprising:
a metal substrate;
a layer of ceramic thermal barrier coating material disposed over the metal substrate, the ceramic thermal barrier coating material effective to thermally insulate the metal substrate from a high temperature fuel/air mixture; and
the ceramic thermal barrier coating material comprising a base compound comprising a crystal structure populated with base elements and further comprising selected sites of the crystal structure being populated by substitute ions selected to allow the ceramic thermal barrier coating material to catalytically react the fuel-air mixture at a higher rate than would the base compound without the ionic substitutions.
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This application is a continuation-in-part of U.S. patent application Ser. No. 09/963,283 filed on 26 Sep. 2001, which is incorporated by reference herein.
This invention was made with United States Government support through Contract Number DOE-DE-FC26-03NT41891 awarded by the Department of Energy, and, in accordance with the terms set forth in that contract, the United States Government may have certain rights in the invention.
This invention relates generally to the field of catalytic combustion, and more specifically to catalytic combustion in a gas turbine engine environment.
In the operation of a conventional gas turbine engine, intake air from the atmosphere is compressed and heated by a compressor and is caused to flow to a combustor, where fuel is mixed with the compressed air and the mixture is ignited and burned. The heat energy thus released then flows in the combustion gases to the turbine where it is converted into rotary mechanical energy for driving equipment, such as for generating electrical power or for running an industrial process. The combustion gases are then exhausted from the turbine back into the atmosphere. These gases include pollutants such as oxides of nitrogen, carbon monoxide and unburned hydrocarbons. Various schemes have been used to minimize the generation of such pollutants during the combustion process. The use of a combustion catalyst in the combustion zone is known to reduce the generation of these pollutants since catalyst-aided combustion promotes complete combustion of lean premixed fuels and can occur at temperatures well below the temperatures necessary for the production of NOx species. Typical catalysts for a hydrocarbon fuel-oxygen reaction include platinum, palladium, rhodium, iridium, terbium-cerium-thorium, ruthenium, osmium and oxides of chromium, iron, cobalt, lanthanum, nickel, magnesium and copper incorporated in a ceramic matrix.
The operating environment of a gas turbine is very hostile to catalytic reactor materials, and is becoming even more hostile as the demand for increased efficiency continues to drive firing temperatures upward. Ceramic substrates used for catalytic reactor beds are prone to failure due to thermal and mechanical shock damage. Furthermore, ceramic substrates are difficult to fabricate into complex shapes that may be desired for catalyst elements. Metal substrates have been used with some success with current generation precious metal catalysts at temperatures up to about 800° C. Such catalytic reactors are produced by applying a ceramic wash-coat and catalyst directly to the surface of a high temperature metal alloy. In one embodiment, the catalytic reactor 12 of
The invention is explained in following description in view of the drawings that show:
Traditional catalytic systems incorporate an active precious metal catalyst such as palladium on a γ-Al2O3 washcoat. The present inventors have found such systems to exhibit poor phase stability, surface area loss, and rapid surface diffusion causing catalyst agglomeration at the very high temperatures desired for modern gas turbine engine designs. For example, the γ-Al2O3 phase having a specific surface area (SSA) value of 125-250 m2/g transforms to either θ or δ phase with an SSA value of 18-30 m2/g at 450° C., which then transforms to α phase with an SSA value of 5 m2/g between 900-1100° C. To solve these problems, the present inventors have innovatively modified ceramic thermal barrier coating (TBC) materials that are known to exhibit acceptable high temperature insulating characteristics with ionic substitutions that serve to improve the catalytic activity of the materials. In certain embodiments, the inventors have also incorporated precious metal crystallites into the ceramic matrix in order to provide low light-off temperature capability for the materials.
The application of a catalytic material to a ceramic thermal barrier coating on a metal substrate is illustrated in
A layer of a ceramic thermal barrier coating material 34 is applied over the substrate, for example on the outside surface of the tube 32. A substrate for a catalyst should exhibit a large surface area for maximizing the contact between the catalyst and the fuel-air mixture passing over the substrate surface. Typical ceramic wash-coats used as catalyst substrates possess a specific surface area (SSA) of approximately 18-30 m2/g. A plasma spray process may be used to deposit the thermal barrier coating 34 as a layered structure with surface connected porosity wherein the pore surface area is purposefully maximized to provide an effective SSA value of greater than 30 m2/g in order to optimize surface catalytic activity. In order to maximize its exposed surface area, thermal barrier coating material 34 may be deposited onto the metal tube 32 by a vapor deposition process in order to produce a columnar-grained microstructure having a plurality of closely spaced columns of material. Such known vapor deposition processes include electron beam physical vapor deposition (EB-PVD), chemical vapor deposition (CVD), electrostatic spray assisted vapor deposition (ESAVD) and electron beam directed vapor deposition (EB DVD). The deposition process parameters may be controlled to optimize the resulting surface area. The columnar-grained structure is known in the art to provide a significant amount of open porosity on the exposed surface of the thermal barrier coating. An idealized EB-TBC columnar-grained thermal barrier coating structure may have an SSA of greater than 30 m2/g, such as between 30-50 m2/g, or between 30-150 m2/g, or between 50-150 m2/g, or between 100-150 m2/g in various embodiments. In one embodiment the structure may have columns of approximately 10 microns diameter and 10 microns height covered with much smaller cones of material of approximately 1 micron diameter and 1 micron height. Although the actual SSA of a thermal barrier coating deposited by EB-PVD has not been empirically measured by the present inventors, it is assumed that the actual usable specific surface area of a controlled EB-PVD coating would exceed that of a ceramic wash coat substrate because the idealized surface area is so large.
The thermal barrier coating 34 may be deposited onto the tube 32 to any desired thickness, in one embodiment to a thickness of about 0.020-inches. A bond coat 36 may be used between the substrate 32 and the thermal barrier coating 34. Common bond coat materials 36 include MCrAlY, where M denotes nickel, cobalt, iron or mixtures thereof, as well as platinum aluminide and platinum enriched MCrAlY. Techniques for applying ceramic thermal barrier coatings over high temperature metal alloys for use in the environment of a gas turbine combustor are well known in the art, so the catalytic element 30 of
Ceramic material 34 functions as both a thermal barrier coating (TBC) material and as a combustion catalyst for supporting combustion at its exposed surface 38. Precious metal crystallites 40 may be incorporated into the ceramic material 34 to reduce the light-off temperature of the material. Material 34 is formed of a crystal structure populated with base elements that may include:
pyrochlores with the formula A2B2O7 where A is selected from the rare earth elements and B is selected from the group of zirconium, hafnium, titanium, niobium and tantalum (for example, La2Hf2O7 and Sm2Zr2O7);
garnets with the formula A3Al5O12 where A is a 3+ cation selected from the group of rare earth elements or transition elements; and
spinels with the formula AB2O4 where A is selected from the group of alkaline earth elements and B is selected from the group of aluminum, iron, manganese, cobalt, chrome and nickel.
Pyrochlore embodiments of the present invention include specially doped A2B2O7 materials as well as Y2O3—ZrO2—TiO2. Pyrochlore systems have been successfully used as TBCs, thus demonstrating their high temperature stability, thermal shock resistance and sintering resistance. The pyrochlore oxides have a general composition, A2B2O7, where A is a 3+ cation (Al, Y, Ga, Sc or rare earth elements from the group including La, Ce, Pr, Nd, Pm, Sm, Eu, Gd, Th, Dy, Ho, Er, Tm or Yb) and B is a 4+ cation (zirconium, hafnium, titanium, etc.). The activity of these systems can be further improved by substituting part of the A site elements or B site elements with other cations. The modified A site can be represented by the formula A2-xMxB2O7 (0<x<1), where M can be any (other than A) 3+ rare earth element or 3+ cation smaller than A such as Al, Y, etc.; or M may be a 2+ cation of the group of Ca, Mg, Sr, and Ba for increased activity. The modified B site can be represented by the formula A2B2-xMxO7 (0<x<1) where M can be a 3+ cation (Al, Sc) or a 5+ cation (Ta or Nb). The other embodiment of the invention in this family is the conventional yttria stabilized zirconia TBC with TiO2 additions. The concentration of the TiO2 may be from greater than 0% to as high as 25 mole %, for example. This system has three advantages: a) it allows for a crystal structure change from fluorite to pyrochlore depending on the composition of the material; b) substitution of the larger Zr4+ with a smaller Ti4+ remarkably increases its ionic conductivity; and c) the titanium ions are able to hop from Ti4+ to Ti3+, thus increasing the catalytic activity of the compound.
Garnet ceramics are being considered for high-temperature structural applications for their superior high-temperature mechanical properties, excellent phase/thermal stability up to the melting point (approximately 1970° C.) and high thermal expansion coefficient (low expansion mismatch with metal substrates). Garnets have a general composition of A3B5O12, where A is a rare earth element (La, Ce, Pr, Nd, Pm, Sm, Eu, Gd, Th, Dy, Ho, Er, Tm, Yb) or yttrium and B is a 3+ cation (Al, Y, Ga, Sc). The catalytic activity of these systems is further improved in the present invention by substituting part of the A site or B site elements with other cations. The modified A site can be represented by the formula Y3-xMxB5O15 (0<x<3) where M can be a rare earth element other than A or another 3+ cation (Ga, Sc). The modified B site can be represented by the formula Y3Al5-xMxO15 (0<x<5) where M can be 3+ cation (Ga, Sc). In another embodiment the substitution of aluminum with iron has the advantage of iron hopping from Fe2+ to Fe3+, thus partly occupying the octahedral or tetrahedral sites. This can be represented by Y3Al5-xFexO15 (0<x<2). Another embodiment is partially substituting Al3+ sites with 2+ cations (Mn2+) or 4+ cations (Ti4+). This remarkably increases the ionic conductivity of the material. This can be represented by Y3Al5-xMxO15 (0<x<2) where M is Mn or Ti.
Spinel ceramic materials generally offer a desirable combination of properties for use in high temperature applications. Magnesium aluminate spinel (MgAl2O4) in particular is considered for thermal barrier coating applications due to its high melting temperature (2135° C.), good chemical stability and mechanical strength. This material has also been widely studied as a catalyst support for catalytic steam reforming of methane due to its low acidity and sintering-resistance ability. The present inventors have found that the catalytic activity of this material can be altered through ionic substitution/doping to meet low light-off/high conversion requirements for gas turbine combustor applications. Spinels have a general composition AB2O4, where A is a site with either tetrahedral (normal spinel) coordination or octahedral/tetrahedral (inverse spinel) coordination, and B is a site with octahedral coordination. Through the substitution of A and B sites with other cations, compositions are possible with improved thermal stability and catalytic activity, such as by partially substituting partial Al3+ sites with 2+ cations (Mn2+) or 4+ cations (Ti4+). This remarkably increases the ionic conductivity of the material and can be represented by MgAl2-xMxO15 (0<x<1) where M is Mn or Ti.
The addition of precious metal crystallites is desired when a two-stage catalyst can be realized in a single stage where the coating on the substrate exhibits enough catalytic activity to satisfy requirements in terms of light-off, conversion and performance. Precious metal crystallites may be incorporated within the crystal structure to allow the ceramic thermal barrier coating material to catalytically react a fuel-air mixture at a lower light-off temperature than would the ceramic thermal barrier coating material without the precious metal crystallites. The precious metal may be incorporated through incipient wetting, where the coating is dipped into precious metal salt to achieve desired loading, or through co-spraying with the ceramic coatings. A precious metal loading of 3-30 mg/in2 may be desired to meet the catalyst requirements for gas turbine engine applications.
While various embodiments of the present invention have been shown and described herein, it will be obvious that such embodiments are provided by way of example only. Numerous variations, changes and substitutions may be made without departing from the invention herein. Accordingly, it is intended that the invention be limited only by the spirit and scope of the appended claims.